1. Field of the Invention
The present invention relates generally to electric power steering apparatuses for use in automotive vehicles which provide an electric steering assist of an electric motor to the vehicle steering system to reduce a steering effort that is to be manually applied by a vehicle driver. More particularly, the present invention relates to an improved electric power steering apparatus which allows the steering wheel to return to its center position at an increased speed as the vehicle driver releases (takes his or her hands off) the steering wheel after excessive turning of the same.
2. Description of the Related Art
Generally, the electric power steering apparatuses for automotive vehicles are designed in such a way that when the vehicle driver releases the steering wheel after turning the steering wheel to change the direction of the vehicle, the tires are subjected to force (self-aligning torque) that acts to restore the tires to the straight-ahead position and thereby cause the steering wheel to be automatically returned to the center position.
However, it is known that as the steering wheel is turned more sharply during travel of the vehicle, the vehicle tends to turn less sharply than the vehicle driver intends (i.e., the vehicle exhibits a stronger understeering tendency) and it becomes more difficult for the steering wheel to automatically return to the center position when released. Such an understeering tendency occurs due to the fact that the self-aligning torque decreases as the slip angles of the tires on a road surface become greater. Particularly, the understeering tendency would appear notably when the road surface has a low friction coefficient (.mu.) (and hence is slippery) or when the steering wheel is unintentionally turned to an excessive extent due to a light steering effort permitted by the electric power steering apparatus.
When the steering wheel is turned to an excessive extent on a slippery road surface, the automatic returning motion of the steering wheel after the driver releases the wheel is significantly retarded due to the reduced self-aligning torque, which would undesirably lead to a disturbance in the vehicle's traveling trail.
FIG. 11 hereof is a graph showing a typical relationship between a steering angle .delta. and torque T.sub.H applied from the tires to the steering wheel as the steering wheel is turned by the driver (this torque T.sub.H will hereinafter be called "tire-to-steering-wheel torque"). In FIG. 11, the tire-to-steering-wheel torque T.sub.H first becomes greater as the steering angle .delta. is increased by the vehicle driver turning the steering wheel outward (to the left or right) from the center position. After the steering angle .delta. exceeds a steady-state threshold value .delta..sub.K and enter a region representing an excessively-turned state of the steering wheel, however, the tire-to-steering-wheel torque T.sub.H decreases, as indicated by a solid-line curve. Subsequently, when the vehicle driver releases the steering wheel at a steering angle .delta..sub.Y, the steering wheel returns to the center position gradually, rather than immediately, because the tires apply very small tire-to-steering-wheel torque T.sub.H, as indicated by a broken-line curve. Then, once the steering angle .delta. gets smaller than the steady-state threshold value .delta..sub.K, the steering wheel returns at a normal rate.
Region (or area) of the tire-to-steering-wheel torque T.sub.H defined by the turning-motion characteristic curve and returning-motion characteristic curve of the steering wheel represents the intensity of the steering-wheel returnability; that is, a smaller area of the tire-to-steering-wheel torque T.sub.H defined by the turning-motion characteristic curve and returning-motion characteristic curve of the steering wheel represents a greater steering-wheel returnability.
If the steering-wheel returnability becomes very small after the steering angle .delta. exceeds the steady-state threshold value .delta..sub.K, a longer time would be required for the steering wheel to return to the center position. Thus, the conventional electric power steering apparatuses present the problem that the traveling trail of the vehicle would undesirably expand outward, resulting in a disturbance in the vehicle behavior.